Complexes of hexamethylenetetramine with zinc-tetraarylporphyrins, and their assembly modes in crystals as clathrates and hydrogen-bonding network polymers

Abstract

This study reveals self-assembly patterns of the tetra(4-pyridyl), tetra(4-hydroxyphenyl) and tetra(4-carboxyphenyl) zinc porphyrin moieties when reacted with a hexamethylenetetramine ligand, forming 1 ∶ 1 five-coordinate, 1 ∶ 2 and 1 ∶ 2 six-coordinate complexes, respectively. Detailed structural characterization of the supramolecular organization in the resulting ordered solids is reported. Crystals of the 1 ∶ 1 complex (1) represent a common tetraarylporphyrin clathrate found earlier in the “porphyrin sponges”. Those of the 1 ∶ 2 complexes consist of multiporphyrin polymeric networks that are sustained by extensive hydrogen bonding, involving the four hydroxylic (in 2) or carboxylic (in 3) functional substituents as proton donors and the N-sites of the hexamethylenetetramine ligand as proton acceptors. The polymeric frameworks are characterized by three-dimensional (in 2) or two-dimensional (in 3) open architectures with wide interporphyrin voids; yet, they do not interpenetrate into one another, giving rise to the formation of open channels that perforate the corresponding crystals and are loosely occupied by guest species of the crystallization solvent. The pronounced effect of the hexamethylenetetramine ligand on the supramolecular aggregation is highlighted, by relating the current findings to previously designed networks of the porphyrin building blocks in question.